[M120 Migration] Fix Terms and Condition wizard shows transparent/white bg

[M120 Migration] Fix Terms and Condition wizard shows transparent/white bg

As the viz strategy is totally updated, for bg logic, we need to flow below:

1. Set bg for Render CompositorFrame
EWebView::SetDrawsTransparentBackground -> WebFrameWidgetImpl::SetDrawsTransparentBackground
-> layer_tree_host -> CompositorFrameSinkImpl -> CompositorFrame with transparent bg
2. Set bg for Browser CompositorFrame
EWebView::SetDrawsTransparentBackground ->
RenderWidgetHostViewBase::SetBackgroundColor
2.1 RenderWidgetHostViewAura::UpdateBackgroundColor()
Compositor::SetBackgroundColor
layer_tree_host -> RootCompositorFrameSinkImpl -> CompositorFrame with transparent bg
2.2 RWHVAuraOffscreenHelperEfl::PaintTextureToSurface
RenderWidgetHostViewBase::GetBackgroundColor()
if set transparent bg, need to enable blend.

References:
https://review.tizen.org/gerrit/291210
https://review.tizen.org/gerrit/301497
https://review.tizen.org/gerrit/302739

Change-Id: I1b0650d5446917c6171df97eebef36c2f947f5fc
Signed-off-by: qiang.ji <qiang.ji@samsung.com>
Signed-off-by: Gajendra N <gajendra.n@samsung.com>